Cyanoacrylate adhesives based on esters of .alpha.-cyanoacrylic acid have gained wide acceptance in recent years for a broad range of industrial and consumer uses. The cyanoacrylate esters themselves, however, are very low viscosity liquids which makes the use of unfilled compositions difficult. Such compositions often migrate from the bondline or are absorbed into porous surfaces. Accordingly, there have been developed a variety of thickened cyanoacrylate adhesive compositions which incorporate organic polymers and/or inorganic fillers to reduce adhesive migration. Examples of such compositions are described in U.S. Pat. Nos. 3,607,542; 3,896,077, 4,105,715, 4,180,913 and in Chemical Abstracts 89:117907c; 89:216475u; 91:40425c; and 92:95114b.
In the art of thickened cyanoacrylates it is sometimes desirable that the composition display thixotropic properties. Thus, under high shear conditions the composition can be processed and easily applied to the substrate but once applied, will display significantly reduced migratory tendencies. It is also well known that the inclusion of fumed silicas in many organic liquid compositions positions produces thixotropic effects. The use of such silicas has been reported in cyanoacrylate compositions.
At least certain of the prior art cyanoacrylate compositions employing fumed silicas have displayed stability problems, however. Thus U.S. Pat. No. 3,607,542 describes organically filled cyanoacrylate compositions in which fumed silica is an optional ingredient. These compositions are reportedly stable for only up to 4 hours. Furthermore, the same patent states that silica by itself does not readily mix with cyanoacrylate monomer to form a paste.
Cyanoacrylate formulations also occasionally find non-adhesive applications. Thus, for instance, the vapors of methyl and ethyl cyanoacrylates have found use for developing latent fingerprints in law enforcement applications. For such applications it would be desirable to develop a nonflowable form of cyanoacrylate monomer so that small open containers of the monomer can be placed throughout a room or automobile to release vapors without the danger of accidental monomer spillage. Desirably the thixotropic additives will also be kept to a minimum so as not to substantially reduce the monomer vapor pressure.
In certain applications it has been discovered that hydrophobic silicas produced by treatment of fumed silica with dimethyldichlorosilane can be used to give thixotropic cyanoacrylate compositions with improved stability. However, these fillers add cure retarding strong acid to the cyanoacrylate composition. Also, the thixotropic ratio of these silicas in cyanoacrylate esters (the ratio of apparent viscosity is measured under specified high and low shear conditions) is quite low. Therefore, when very high thixotropic effects are desired, such as when a pasty composition is desired, the amount of acid introduced by the silica can substantially reduce the cure time of the composition. Also, for latent fingerprint developing type applications, higher silica levels may result in lower monomer vapor pressures.
Commercially available fumed silicas are also known which have been treated with hexamethyldisilazane. It has discovered that thixotropy ratios are also very low for these materials and that at least some of these materials tend to destabilize cyanoacryalte compositions. The destabilization effect is thought to result from residual ammonia or amine in the filler.
Accordingly there exists a need for a cyanoacrylate thixotrope, stable to the cyanoacrylate monomer, which has a significantly improved thixotropy ratio over dimethyldichlorosilane or hexamethyldisilazane treated silicas, and which does not adversely affect the fixture time of adhesive formulations.